The present invention relates generally to a method of disinfecting medical materials and more particularly to a method of disinfecting medical materials by exposing the materials to a combination of heat and gamma radiation. The term medical materials encompasses medical waste, veterinary waste and medical products. The problems with current waste handling methods will be discussed first.
The problem of disposal of solid waste is becoming increasingly acute. The primary methods of solid waste disposal have been burning or burial in landfills. These two methods have severe disadvantages. Burning liberates waste particles and fumes which contribute to acid rain. Burying wastes results in toxic chemicals leaking into the surrounding earth and contaminating the water supply. Although increasing amounts of solid waste are being recycled, which alleviates the problems of the other two disposal methods, presently available recycling methods do not provide a complete solution to the disposal problem.
Waste disposal is of even more urgent concern when the waste may cause infection. Such infectious waste is a by-product of medical and veterinary care. For example, regulated medical waste consists of the following categories:
1. Cultures and stocks of infectious agents and associated biologicals, PA0 2. Pathological wastes, PA0 3. Human blood and blood products, PA0 4. Contaminated sharps (including needles, syringes, blades, scalpels, and broken glass), PA0 5. Animal waste, PA0 6. Isolation waste (gloves and other disposable products used in the care of patients with serious infections), and PA0 7. Unused sharps.
These wastes can be generally divided between general medical waste, including waste listed above in categories 1, 2, and 3; veterinary waste, or category 5; and waste that is predominantly plastic, including categories 4 and 6. Hospitals typically segregate types of waste. Contaminated sharps and isolation waste are categories of special concern, as this waste may carry highly dangerous infections such as AIDS or hepatitis. Sharps in particular have caused public panic when observed on beaches and other public areas.
Hospitals and other generators of medical and veterinary waste employ three main methods of waste handling: 1) on-site incineration of the waste, 2) on-site steam autoclaving of the waste and later shipment to a landfill, and 3) no on-site processing before turning the waste over to a waste hauler.
Predominantly located in urban areas, many hospital incinerators emit pollutants at a relatively high rate. In the emissions of hospital incinerators, the Environmental Protection Agency (EPA) has identified harmful substances, including metals such as arsenic, cadmium, and lead; dioxins and furans; organic compounds like ethylene, acid gases, and carbon monoxide; and soot, viruses, and pathogens. Emissions from these incinerators may be a bigger public health threat than improper dumping. (Stephen K. Hall, "Infectious Waste Management: A multi-faceted Problem," Pollution Engineering, 74-78 (Aug. 1989)).
Although steam autoclaving may be used to disinfect waste before further processing, it is expensive and time-consuming. Temperature monitoring devices such as thermocouples and biological indicators such as heat-resistant Bacillus stearothermophilus spores may be used to assure effective disinfection. The application of heat denatures the protein in microorganisms causing death in a short time. Viruses are rapidly inactivated; bacteria and particularly bacterial spores survive somewhat longer than viruses.
U.S. Pat. No. 2,731,208 (Dodd) teaches a steam-sterilizing apparatus for disposing of contaminated waste which incorporates shredding the waste ("including paper containers such as used sputum cups," Col. 1, Lines 28-29). This reference teaches processing only limited types of items; it teaches the use of steam sterilization alone and has the further disadvantage of depositing the shredded mixture into a sewer. (Col. 4, line 49).
Whether or not the hospital first autoclaves its medical waste, including broken needles and glass, the waste is then turned over to a waste handler for transport to a landfill or other depository. U.S. Pat. No. 3,958,936 (Knight) teaches compaction of hospital waste for more efficient landfill disposal. Specifically, this reference teaches the application of heat in the range of about 400.degree. to 600.degree. F. to hospital and other waste to melt the plastic and turn it into a hard, compact block for safer disposal in landfills. The waste is disinfected and needles become imbedded in the plastic. This method has the disadvantages of requiring high temperatures and landfill disposal. As mentioned above, metropolitan landfills are becoming filled and unauthorized dumping is becoming a problem.
Another area of concern is the sterilization of medical products. By medical product we mean any product which must be disinfected or sterilized prior to use in patient or animal care. This area is exemplified by, but not limited to, the following: needles, syringes, sutures, scalpels, gloves, drapes, and other disposable items. Many reusable items also must be provided in sterile form. Primary sterilization methods include the use of autoclaving, ethylene oxide, and ionizing radiation. The heat and humidity of autoclaving are quite damaging to many disposable medical products; hence autoclaving is not preferably used, and ethylene oxide and ionizing radiation are preferred commercially.
To sterilize medical products with known methods, poisonous ethylene oxide gas fills a closed chamber containing the products to be sterilized. For effective sterilization, not only must the ethylene oxide concentration be carefully controlled, but the temperature, humidity and porosity of the sterilizer load also must be regulated. Ethylene oxide is slow to dissipate from plastics and may require that the medical products be stored until the ethylene oxide falls to a safe level. Ethylene oxide also must be carefully vented to the atmosphere after the sterilization cycle to avoid poisoning workers.
If ionizing radiation such as gamma radiation is used by itself, it must be administered at such intense doses that many plastics become yellow and brittle. For example, U.S. Pat. No. 3,940,325 (Hirao) teaches ways to adjust the formulas of plastics for syringes to avoid yellowing and cracking after exposure to gamma radiation. Other substances may also be damaged by radiation.
Ionizing radiation, or gamma radiation, is produced by electron accelerators or radioisotopes such as cobalt 60 or cesium 137. Both sources produce high-energy photons which disinfect by inactivating the DNA of viruses and bacteria. These irradiated microorganisms lose their ability to reproduce and cause infections. Gamma radiation rapidly inactivates bacteria but is less effective against viruses. On a large-scale industrial basis, gamma irradiation with cobalt 60 has been used to sterilize medical products prior to their use in patients. The dosage of gamma radiation, measured in rads or megarads (Mrads), varies but a dose of 2.5 Mrads is usually selected as a starting point in known methods. However, such doses also damage the product being sterilized. The following patents teach methods to sterilize medical products with less harm to the product.
U.S. Pat. No. 3,617,176 (Clouston) teaches a method of improving sterilization efficiency by increasing hydrostatic pressure. Elevated hydrostatic pressure causes sterilization-resistant bacterial spores to germinate or begin to grow, but it has no effect on viruses. Germination makes the bacteria more sensitive to radiation. This reference teaches optimizing the hydrostatic pressure effect by adjusting temperature (up to 80.degree. C.), and then disinfecting the sutures with lower doses of gamma radiation or other modes of disinfection. According to Clouston, elevated pressure and fluid or moist gas are essential to his method; raised temperature alone has a negligible effect. Furthermore, the pressure/heat/moisture treatment this reference teaches is intended to cause bacterial spores to germinate, not to immediately sterilize or inactivate microorganisms.
In contrast, U.S. Pat. Nos. 4,620,908 (Van Duzer) and 3,704,089 (Stehlik) teach pre-freezing injectable proteins and surgical adhesive respectively before irradiation with cobalt 60. In these methods, the temperature is reduced not to sterilize the product, but to protect the product from damage by gamma radiation.
U.S. Pat. No. 3,602,712 (Mann) describes an apparatus for gamma irradiation and disinfection of sewage and industrial waste. Gamma radiation by itself, however, is impractical for disinfecting medical waste. Gamma radiation in the doses used to sterilize medical products is considered too expensive for medical waste processing.
Besides gamma radiation, other energy sources are being considered as potential sterilants in known systems. Microwaves are increasingly being investigated for rapid sterilization of individual medical devices and shredded medical waste. Recently, an experiment showed that metallic instruments could be disinfected in only 30 seconds in a microwave. (N.Y. Times, "Science Watch: Microwave Sterilizer is Developed," June 20, 1989). A problem is that this particular method can handle only a few instruments at a time.
According to one publication, a medical waste disposal system utilizing microwaves has apparently been developed. This system first shreds the waste, sprays it with water and passes the mixture through a microwave chamber designed to raise the temperature of the mixture to 205.degree. C. After the disinfection step, the system compresses the waste and packages it for shipment to landfills or incinerators. (The Wall Street Journal, p. B3, Apr. 10, 1989). One potential problem with this system is that shredding before disinfection could release infectious particles to the environment and may thus spread contagion. Another problem is ultimate disposal of the waste: It persists in landfills or may pollute the air when incinerated.
Further, microwaves are limited in their penetration. If applied to large-scale, boxed medical waste, the microwaves alone do not heat very effectively. In contrast, radio-frequency (R-F) waves are relatively low-frequency waves which penetrate more effectively. Radio-frequency waves have been used directly and indirectly for sterilization.
U.S. Pat. No. 2,114,345 (Hayford) teaches a radio-frequency applicator with electroscopic control for destroying bacteria in bottled beer and similar articles. This reference teaches an apparatus that sterilizes with radio-frequency waves alone. Therefore, it teaches away from the combination of radio-frequency waves with gamma radiation.
U.S. Pat. No. 3,948,601 (Fraser et al.) teaches the indirect use of radio-frequency waves in disinfecting a wide variety of medical and hospital equipment as well as human waste. This reference teaches the use of radio-frequency waves to heat certain gases (particularly argon) to ionize into gas plasma at approximately 100.degree. to 500.degree. C. This references teaches that "cool" plasma, (Col. 1, Line 12) reaches the article to be sterilized at a temperature of only 25.degree. to 50.degree. C. and very low pressure and effectively sterilizes the article. However, sterilization by plasma gas, does not suggest the direct use of radio-frequency waves in sterilization
Reprocessing of waste and especially medical waste is vital for several reasons. First, landfills, particularly in many urban areas, are becoming filled. In addition, older landfills may leak. Thus, burying wastes is becoming more of a problem. Second, merely burning waste can pollute the atmosphere and cause acid rain. Current reprocessing technology should be employed to process medical waste for effective utilization.
What was needed before the present invention was a method to disinfect or destroy the infectious potential of medical waste and to dispose of it in a manner harmless to health care workers, waste handlers, and the public at large.